1. Field of the Invention
The invention relates generally to a process and an apparatus for the conversion of water vapor coal mixture or water vapor hydrocarbon mixture to form a product gas containing hydrogen, and specifically to the production of water gas or synthesis gas by methane-steam splitting and steam-coal gasification.
The process of conversion of CH.sub.4 and H.sub.2 O vapor to CO, H.sub.2 and CO.sub.2 is widely known in prior art, and, in traditional steam converters, takes place at 700.degree. to 850.degree. C. in a reaction chamber and at pressures between 20 and 40 bar; the reaction generates a product gas which contains H.sub.2, CO, CO.sub.2, H.sub.2 O and approximately 8 to 13% by volume of unconverted CH.sub.4.
As is well known, the CH.sub.4 /H.sub.2 O reaction is endothermic, and needs a supply of heat. The strongly endothermic CH.sub.4 /H.sub.2 O conversion is made to take place in externally-heated tubes with a desired inside diameter, e.g., approximately 90 to 130 mm in the presence of catalysts containing nickel at thermodynamic equilibrium conditions. In prior art, there is a substantial amount of unconverted residual methane--30 to 40% of the entry duct methane--which is generally undesirable for the subsequent reactions of the gas. In many cases the high residual methane content is downright detrimental, so that in current installations, a further conversion of the remaining methane with oxygen at approximately 1200.degree. C. in a secondary converter or reactor is resorted to. After completion of the reaction in such a secondary converter, the methane concentration of the reaction mixture is less than 1%. Additional subsequent process stages may include, in addition to secondary conversion, CO.sub.2 washing and, if necessary, low-temperature decomposition, before the synthesis gas can be conducted to the use for which it is intended.
2. Description of the Prior Art
Hydrogen permeable membranes of various metals, alloys and permeable walls of varied structural configurations are known in the prior art for separation and extraction of hydrogen gas from gaseous mixtures and product gases containing hydrogen. Certain related published patents/documents and pending applications concerning hydrogen permeable membranes are discussed hereinafter.
German Laid Open patent application No. DE-OS 28 23 521 generally teaches design and other structural considerations for selectively permeable membranes for hydrogen diffusion. Also discussed therein in general terms are different degrees of hydrogen diffusion in membranes made of different alloys/metals. Specifically, this German Patent teaches the use of an assembled bundle of a plurality of fine tubular ducts made of a hydrogen permeable material, for example, Nickel or Nickel alloy. The outside of each tubular duct is sealed into a wall of a chamber. There is no discussion or teaching in this prior art publication of the effects of differential pressures and the effects of proximity of a catalyst to the membrane, on the hydrogen diffusion through the membrane. The above-cited German Laid Open patent application is incorporated herein by reference.
German Laid Open patent application No. DE-OS 28 54 638 and the disclosure in pages 304 and 305 of the publication "Grundlagen der allgemeinen und anorganischen Chemie" generally deal with respectively the diffusion of Tritium and general chemical reactions without specific reference to the problem which the present invention addresses. More specifically, the German Laid Open patent application No. DE-OS 28 54 638 teaches regarding an arrangement comprising first and second adjacent chambers, both having movable walls, the chambers being separated by a hydrogen permeable wall. However, German Laid Open patent application No. DE-OS 28 54 638 does not contribute to the concept of the present invention wherein the conversion of a carbon containing substance into a product gas is augmented by continuous hydrogen extraction. The above-cited German Laid Open patent application is incorporated herein by reference.
U.S. patent application Ser. No. 649,043, incorporated herein by reference, teaches a hydrogen permeable membrane in the form of a metal coating such as palladium and alloys of palladium less than 100 microns thick and supported by a fabric with a mesh width not exceeding 50 microns; alternatively, the membrane may be formed of palladium with an adjacent gas permeable support structure which could take any convenient form, for. example, spiral springs, as is taught in German Patent No. DE-PS 14 67 079, also incorporated herein by reference.
U.S. patent application Ser. No. 648,898, incorporated herein by reference, teaches a manufacturing process for a hydrogen permeable membrane comprising a palladium wall or a palladium-silver wall. Several examples of metals such as Nb, Ta, Pd and Zr which have hydrogen permeation properties are discussed. Also disclosed are examples of certain types of glasses and plastics which have hydrogen permeation properties. In a preferred example described therein, a hydrogen permeable metal is galvanically deposited on a fine mesh of metal fabric.
U.S. Pat. No. 3,957,534, issued on May 18, 1976 to Linkohr et al, teaches a hydrogen diffusion diaphragm which selectively separates hydrogen from hydrogen-containing gaseous mixtures, and is incorporated herein by reference. The diaphragm comprises an alloy of nickel and titanium in such proportions as to contain more than 50 molar percent of nickel. The dependence of the diffusion co-efficient of the diaphragm on the dimensional parameters like the diaphragm thickness is discussed. There is no reference however, in this prior art patent to any need for improving the efficiency of operation in a steam-hydrocarbon conversion process to generate a desired product-gas, as in the present invention. As described herein, the present invention is concerned with improving the conversion efficiency for obtaining a product gas from a steam-hydrocarbon reaction without the need for additional supplementary operation of the prior art.
U.S. Pat. No. 3,901,668 issued on Aug. 26, 1975 to Seitzer teaches a process for obtaining oxygen from steam by subjecting the steam to a temperature above 1500.degree. C. to dissociate the steam into oxygen and hydrogen; the dissociated steam is passed through a chamber which has a hydrogen permeable wall to extract hydrogen, and is incorporated herein by reference. The gaseous effluent from the process is cooled to obtain an oxygen rich stream. This prior art U.S. Patent to Seitzer is not directed to improving the conversion efficiency for conversion of steam-hydrocarbon mixture into a product gas; there is no mention either, in this prior art patent regarding the effect of prompt and fast evacuation of the generated hydrogen, on the entire process.